Page 325 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

FOOD PROCESSING 303

4.30.3 External Coatings/Paint
Painting the exposed external surfaces of equipment gives good appearance as well
as corrosion protection. The selection of the type of coating depends on the use of
the equipment. External coatings must withstand the effects of abrasion, weather, and
ultraviolet radiation. Surface preparation and painting is an easy method to prevent
or delay the onset of corrosion.
For underground structures such as USTs, corrosion control of the external sur-
faces can be achieved with a combination of CP and a dielectric coating. The external
coating must be applied when the tank is new. A buried tank cannot be retrofitted with
an external coating unless it is removed from the ground.

4.30.4 Internal Linings

Internal corrosion protection, where required because of contamination or corrosive
products in storage tanks, is usually maintained with an internal liner sometimes in
combination with galvanic CP. Internal coatings prevent internal corrosion and pro-
long operational life of the tanks. An example of this is the internal lining in mild steel
fertilizer tanks. The linings offer corrosion protection from fumes and condensation
in the vapor space and immersion exposure to the stored liquid chemicals.

4.30.5 Cathodic Protection
Aboveground tanks containing stored liquid and is contaminated with water layer
should be internally coated and cathodically protected on the bottom and partially
along the wall. The external bottom corrosion of the site-fabricated tanks can be con-
trolled with select sand/concrete foundation pads, impervious liners, and CP.
The design of CP for new ASTs and USTs includes consideration of the proximity
to other metallic structures and existing CP systems, the type of grounding, the
estimated remaining life of the tank, the type and temperature of the stored product,
the amount of product stored, the cycling rates, the method of tank bottom plate
construction, the type of tank foundation, back-fill soil characteristics. The two
types of sacrificial anodes are (i) zinc or magnesium ribbon or ingot anodes, and (ii)
impressed-current CP using perimeter deep-buried, angle-drilled anodes or vertical,
loop, or string undertank anodes.

4.31 FOOD PROCESSING

A variety of materials are used in food processing such as stainless steels and alu-
minum alloys. Plastics and other materials such as brass and bronze may be used.
Plastics and other metals may be used. Lead and cadmium plated materials impart
toxicity to foods. Food contact surfaces must be smooth, no-adsorbent, nonleaching,
and insoluble in the food.
Stainless steels are resistant to corrosion in food-processing environments. How-
ever, stainless steel is not immune to a chloride-containing environment. Corrosion